The classical optical theorem that describes the extinction of electro
magnetic radiation by an arbitrary particle is reformulated by use of
the diffusion properties of the mutual spectral densities. This reform
ulation yields three novel coherency tensors: an extinction-loss coher
ency tensor, a scattering-loss coherency tensor, and an absorption-los
s coherency tenser. The first tensor is used to establish the polarime
tric optical theorem and to derive the extinction-loss matrix. The sec
ond tensor is used to derive the scattering-loss matrix. The third ten
sor is used to develop the absorption-loss matrix and the absorption c
ross sections. Furthermore, the derived extinction-and scattering-loss
matrices are found to be similar to the extinction coefficient and ph
ase matrices of the vector radiative transfer equations. (C) 1998 Opti
cal Society of America.